Harper is off to an excellent start. He began the week leading the National League in home runs and walks, while ranking second in runs, third in RBIs and fourth in on-base percentage. Betts, however, is playing like Secretariat at the Belmont. He began the week leading the majors in home runs, runs, batting average and slugging percentage, while ranking second in WAR.

We all know Harper is going to receive a mega-payday in free agency, given his age -- he’ll turn 26 in October -- and potential to put up MVP-caliber numbers like he had in 2015 (when he won) and in 2017 (when he might have won if he hadn’t hurt his knee in August).

With Mookie Betts' powerful bat and sparkling defense, his WAR rates higher than Bryce Harper's over the past four seasons. AP Photo/Elise Amendola

Let me throw this out there: If Harper is going to receive a $300 million or maybe $400 million contract, what will Betts get? Betts will be eligible for free agency after the 2020 season, and while he’s the same age as Harper -- they were born nine days apart -- that means he’ll be two years older than Harper at free agency, which factors into the potential payout. But consider each player’s total WAR since the start of 2015:

Betts: 24.6

Harper: 17.0

That’s Baseball-Reference WAR, which uses defensive runs saved as its defensive component, and DFS absolutely loves Betts, crediting him with plus-63 runs above average in right field in 2016 and 2017.

OK, here are the FanGraphs numbers:

Betts: 21.1

Harper: 18.5

It's a little closer, but Betts still comes out ahead. Is he the better all-around player? It’s hard to argue against that statement, although the gap for now is because of his big edge on defense and durability (but note that the four seasons we’re looking at include only one season in which Harper missed significant time).

The weird thing about Harper’s 2018 numbers is that he has just two doubles to go with his 12 home runs. Betts, meanwhile, has remained a doubles machine even with all the home runs (13) and is on pace to top 40 doubles for the third straight season. Betts has been the better offensive player in 2018, but Harper holds a decisive edge over the four seasons, as Baseball-Reference estimates he has created 123 runs above average, compared to 73 for Betts.

Harper supporters also might suggest that Betts is helped by a stronger lineup around him. Indeed, Harper has seen just 41.7 percent of pitches in the strike zone, the second-lowest rate in the majors. However, that figure matches his 41.8 percent rate since 2015, so he’s not necessarily being pitched more carefully this season than in the past. Betts also is helped by Fenway Park -- he has hit .314 there, compared to .277 on the road for his career, although he actually hits more home runs on the road. (He hits way more doubles at home, thank you, Green Monster.)

Anyway, it’s a close call. I’d take Betts for 2018, given his premier defense and hot start, which suggests his offense might go to a new level. I might prefer Harper on a 10-year contract, but that’s gambling on his health.

For the third straight season and fourth season in five, Kershaw is going to miss time on the disabled list, this time with biceps tendinitis. Kershaw won his fifth ERA title in 2017, and his ERA since 2013 is 1.99. And it's at 2.86 this season, even as he was pitching with diminished velocity.

The concern isn’t so much his ability, but his ability to stay on the mound. He also has seen his home run rates increase drastically over the past two seasons, perhaps the strongest indicator that Peak Kershaw has come and gone.

Is he still the best? I’d say the torch has been passed -- I’m just not exactly sure to whom. Bill James has a formula to rank starting pitchers, which gives more credit to recent starts while incorporating results from the past, but with less emphasis the further back you go.

A year ago, Kershaw held a 14-point edge over Scherzer at 577 to 563. I think it’s fair to say those four guys all have passed Kershaw -- they’ve all been more durable (Kluber did miss a little time last year with a back issue). I’m good with Scherzer as the current No. 1, especially given his lights-out start in 2018, with 80 strikeouts, a 1.74 ERA and just three home runs allowed in 51⅔ innings.

The Astros could have the best rotation ever

Whoa, let’s not get too far ahead of ourselves. It has been only 36 games. Still, what an amazing start. Houston’s rotation owns a 2.34 ERA, while averaging 10.63 strikeouts per nine innings. That matches Kershaw’s mark for 2014, when he led the majors. At the beginning of the week, Baseball-Reference estimated the rotation’s value at 6.2 WAR and FanGraphs had it at 6.3. Split the difference and prorate it over 162 games and you get 28.1 WAR.

Comparing rotations across eras is even more problematic than comparing position players. In the dead-ball era, starters pitched ungodly numbers of innings. Modern-day pitchers benefit from increased strikeouts. Offensive levels have changed throughout history, making straight ERA comparisons worthless. For what it’s worth, FanGraphs’ top five rotations using WAR:

The interesting thing is how many innings Astros starters have thrown so far. That pace is probably unsustainable, given that Houston starting pitchers threw just 899⅔ innings in 2017. This group is better and different with a full season of Verlander and Gerrit Cole taking his game to another level, but no team's starters have reached 1,000 innings in the past two seasons.

Still, the Astros have put themselves on the map for fielding a potential all-time great rotation. If the pitchers stay healthy, you never know.

I mean, he’s probably not going to keep that up -- Babe Ruth, with 14.1 WAR in 1923, is the only player to reach even 13 WAR, according to B-R. Barry Bonds peaked at 11.9 in 2001. Carl Yastrzemski’s remarkable Triple Crown season in 1967 comes in at 12.5, behind only two Ruth seasons. Trout’s personal best is 10.5 in 2012 and 2016, but given his early power output and improved defensive metrics, a 12-WAR season appears possible. And, yes, that means he’s still the best player in baseball.